1. Field of the Invention
The present invention relates to a process for producing thermoplastic resin pellets, and to an apparatus for producing the pellets. More particularly, the present invention relates to a process for producing thermoplastic resin pellets, which provides pellets of uniform shape, which prevents or reduces generation of industrial waste (e.g., pellets of irregular shape, crushed pieces, powder, or long, miscut strands), and which reduces breakage of a pelletization apparatus (e.g., a molten strand cutter); and to an apparatus for producing the thermoplastic resin pellets.
2. Background Art
When a molten polymer is formed into pellets; for example, when a molten polymer (e.g., polyamide or polyester) is discharged from a polymerization vessel for pelletization, there is widely used a polymer pelletization apparatus in which such a molten polymer is extruded in the form of strands through holes having a specific form (such a hole is called a “die hole” or “nozzle”), followed by solidification, and the polymer strand is cut into pellets by means of a cutter.
Thermoplastic resins to which the aforementioned pelletization method can be applied include polyolefin, polystyrene, polyester, polycarbonate, and polyamide. Particularly, the pelletization method is useful for polymer materials obtained through polycondensation reaction (e.g., polyester and polyamide).
Since polyamide exhibits excellent chemical and mechanical properties (e.g., high strength, wear resistance, fatigue resistance, good stainability, and gas barrier property), it can be used as an injection molding material for, for example, home electric appliances, automotive parts, and computer housings; as a clothing material (e.g., fiber yarn or knitted fabric); as a filament material for industrial or leisure use (e.g., tire cord, fishing net, or fishing line); or as a material for food packaging films, container sheets, or bottles. Particularly, polyamide MXD6 (i.e., polyamide having repeating units including an amide bond formed between an aliphatic dicarboxylic acid, and xylylenediamine or bis(aminomethyl)cyclohexane) exhibits high strength, high elastic modulus, low water absorption, and excellent gas barrier property, as compared with, for example, polyamide 6 or polyamide 66. Therefore, polyamide MXD6 is particularly useful as an industrial material or as a material for food packaging films or sheets or bottles.
Generally, when a molten polymer is discharged from a polymerization vessel, in the case of a batch-type method, the interior of the polymerization vessel is pressurized with an inert gas (e.g., nitrogen gas), whereas in the case of a continuous polymerization method, a gear pump, a screw, or a similar apparatus is used. When pelletization is carried out through such a method, the amount of the molten polymer supplied to a pelletization apparatus is varied with, for example, change over time in viscosity of the polymer in the polymerization vessel or change in extrusion pressure, which causes insufficient cooling due to adhesion between polymer strands.
When pelletization is carried out under such conditions, non-discrete pellets or blocking pellets may be generated. The presence of such irregular-shape pellets causes a problem in that normal pneumatic transportation of the pellets fails to be performed, or the pellets cannot be discharged from a stock silo due to formation of a bridge. When such irregular-shape pellets (i.e., pellets having considerably different shapes or sizes) are used, for example, the user cannot consistently supply the pellets to a molding machine, which is problematic.
In order to solve such problems, generally, there has been employed a method in which pellets of irregular shape are separated and removed from discharged pellets by means of a vibration sieve. However, this method necessarily leads to reduction in yield. Particularly, polyamide (in particular, polyamide MXD6, which is a polymer of high elastic modulus) is relatively likely to be affected by change in conditions of a pelletization process, since strands of the polymer exhibit high hardness.
In some cases, disturbance of flow of molten strands may cause inconsistency in size of the strands or instability in cooling/solidification state, resulting in progress of wear of, for example, a cutter, and an increase in frequency of maintenance. Particularly, a polymer of high elastic modulus (e.g., polyamide MXD6) tends to promote wear of a pelletization apparatus, since strands of the polymer exhibit high elastic modulus. Progress of wear of a pelletization apparatus causes difficulty in cutting of polymer strands, resulting in generation of, for example, crushed pieces or powder. In addition, non-cutting of polymer strands (i.e., formation of long strands) causes stoppage of a pelletization process due to clogging of a pelletization apparatus or overload to the apparatus; i.e., reduction in yield of a final product.
Such irregular-shape pellets, crushed pieces, powder, or long, miscut strands are disposed of as industrial waste, and thus cost for disposal or transportation of the waste is further increased, which results in low profit. In recent years, from the viewpoint of growing concern for the environment, demand has arisen for prevention of disposal of waste. Meanwhile, from the viewpoint of an increase in yield, keen demand has arisen for development of a method for preventing or suppressing generation of such industrial waste.
In view of the foregoing, there has been proposed a method for increasing the yield of a final product to a maximum possible extent, in which the state of strands or the shape of pellets is constantly monitored by an operator, and, when any abnormality is observed, discharge of a polymer is stopped, or a receiver for pellets is changed. However, such a manual method poses problems in that a long period of time is required to deal with such abnormality, large amounts of defective pellets are generated, and labor savings (e.g., automated pelletization) fail to be attained.
In order to solve such problems, JP 04-25408A proposes a method employing an apparatus for automatically determining data on the shape of pellets, and an apparatus for regulating cutting speed on the basis of the thus-determined data. However, in this method, cutting speed is regulated only in the flow direction (machine direction) of strands, and thus difficulty is encountered in satisfying the pellet size requirements of users. In addition, this method poses problems in that a response to rapid change in extrusion pressure is delayed; pellets of non-uniform shape are formed; and pellets of irregular shape are necessarily contained in a final product.
Also, JP 08-164519A proposes a method in which cutting speed is regulated on the basis of the molten polymer viscosity and pressure as determined upon discharge of the polymer, to thereby form pellets of uniform shape. However, this method requires determination of pressure and melt viscosity with high accuracy, and thus requires a complicated structure and high cost. In addition, in this method, take-off speed changes in accordance with change in pressure or viscosity of a molten polymer discharged, and thus difficulty is encountered in reducing or suppressing generation of pellets of irregular shape in the case where strands of a material employed show considerable change in hardness in accordance with change in cooling conditions for the strands.
In the case of a continuous polymerization method, generally, a gear pump, a screw, or a similar apparatus is used so as to discharge a molten polymer at a constant rate in a pelletization process. However, employment of such an apparatus is not desired in the case of a batch-type method, since high equipment investment is required. In addition, unwanted retention part of a molten resin may occur, and thus the retained resin may be deteriorated when a long interval is provided between batch processes or a long period of time is required for a batch polymerization process. JP 11-254431A proposes a method in which the opening degree of a valve for discharging a polymer is regulated on the basis of the determined weight of produced pellets, so that the polymer is discharged at a constant rate, and pellets of uniform shape are produced. However, in this method, change in weight of produced pellets is detected after separation of a cooling medium (e.g., water) from the pellets, and thus difficulty is encountered in rapidly responding to change in amount of a polymer discharged. In addition, in the method for regulating the opening degree of a discharge valve on the basis of change in weight of produced pellets, change in flow of polymer strands may occur directly, and thus difficulty is encountered in reducing or suppressing generation of pellets of irregular shape.